Such a craft, which acts as a small floating dock, is described in document FR 2 863 579.
As described in this document, the craft comprises a ballast arranged in the rear part of the rigid hull. This ballast is associated with pumping means for filling and emptying it as needed.
Filling the ballast with water has the effect of modifying the weight of the hull and therefore submerging said hull as desired, particularly its stern.
To allow introducing a load into the craft, and subsequently releasing it if applicable, the hull does not have a transom. Its stern is open, and loads are moved on its deck using a winch.
In fact, this craft is a simple sled which is moved by another, motorized, craft.
As described in the above document, filling the ballast with water submerges the stern of the craft, and it can be submerged to a greater or lesser extent depending on the type of load to be collected from or placed in the water.
To increase the amount by which the stern is submerged, or in other words to lower the level of the deck, this document FR 2 863 579 also specifies deactivating the rear chambers of the side floats by deflating them.
These two rear chambers can be deflated by means of a valve system. Each chamber is connected to a three-way valve which allows deflating and reinflating it using one or more bottles containing compressed air, such as diving cylinders.
The above document specifies using an electric pump to empty and fill the ballast, and therefore having a battery on board in order to power this pump.
Considering the conditions for using this type of craft, such pumping equipment imposes preparation and maintenance constraints if a reliable and efficient operation is to be ensured under all circumstances.